Re: [PATCH mm-new 1/1] mm/khugepaged: guard is_zero_pfn() calls with pte_present()

[Wei Yang <richard.weiyang@gmail.com>]

On Thu, Oct 16, 2025 at 11:47:06AM +0530, Dev Jain wrote:
>
>On 16/10/25 9:06 am, Lance Yang wrote:
>> From: Lance Yang <lance.yang@linux.dev>
>> 
>> A non-present entry, like a swap PTE, contains completely different data
>> (swap type and offset). pte_pfn() doesn't know this, so if we feed it a
>> non-present entry, it will spit out a junk PFN.
>> 
>> What if that junk PFN happens to match the zeropage's PFN by sheer
>> chance? While really unlikely, this would be really bad if it did.
>> 
>> So, let's fix this potential bug by ensuring all calls to is_zero_pfn()
>> in khugepaged.c are properly guarded by a pte_present() check.
>> 
>> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
>> ---
>>   mm/khugepaged.c | 13 ++++++++-----
>>   1 file changed, 8 insertions(+), 5 deletions(-)
>> 
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index d635d821f611..0341c3d13e9e 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -516,7 +516,7 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte,
>>   		pte_t pteval = ptep_get(_pte);
>>   		unsigned long pfn;
>> -		if (pte_none(pteval))
>> +		if (!pte_present(pteval))
>>   			continue;
>>   		pfn = pte_pfn(pteval);
>>   		if (is_zero_pfn(pfn))
>> @@ -690,9 +690,10 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
>>   	     address += nr_ptes * PAGE_SIZE) {
>>   		nr_ptes = 1;
>>   		pteval = ptep_get(_pte);
>> -		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
>> +		if (pte_none(pteval) ||
>> +		    (pte_present(pteval) && is_zero_pfn(pte_pfn(pteval)))) {
>>   			add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
>> -			if (is_zero_pfn(pte_pfn(pteval))) {
>> +			if (!pte_none(pteval)) {
>
>Could save a level of indentation by saying
>if (pte_none(pteval))
>    continue;
>

Vote for this :-)

No other comment.

Reviewed-by: Wei Yang <richard.weiyang@gmail.com>

-- 
Wei Yang
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